1. Field of the Invention
The present invention relates to methods for the joining, by an aircraft, of a secure zone, with a minimum of constraint in the horizontal plane. It deals especially with the case of aircraft threatened by risk of collision with the ground or by a risk of penetration into a forbidden zone which cannot be resolved by a purely vertical avoidance maneuver.
2. Description of the Related Art
Aircraft are increasingly often being equipped with ground proximity warning systems whose role is to prevent aeronautical accidents in which an aircraft which is still maneuverable crashes with the ground, accidents known in the technical literature by the acronym CFIT standing for the expression “Controlled Flight Into Terrain”. These ground proximity warning systems produce alerts and alarms to attract the attention of the crew of the aircraft to the need to correct the trajectory of the aircraft but leave, most of the time, to the initiative of the crew, the terrain avoidance trajectories, that is to say the joining of a secure zone, with a minimum of constraint in the horizontal plane, such as the space above a safety altitude greater than the highest reliefs of the region overflown.
Essentially two generations of ground proximity warning systems are known, the first known by the name GPWS (the acronym standing for the expression: “Ground Proximity Warning System”) and the second by the name TAWS (the acronym deriving from the expression: “Terrain Awareness Warning System”).
GPWS ground proximity warning systems take no account of the flight conditions of the aircraft and only emit alarm messages of the style “terrain, terrain”, “Terrain Ahead, pull up” for the attention of the crew of an aircraft. They pose a problem of adjustment of sensitivity, a compromise having to be sought between timely triggering with each true risk of collision with the ground and a minimum of false alarms.
TAWS ground proximity warning systems supplement the information taken into account by GPWSs with navigation data and maps of the relief overflown that are extracted from topographical databases on board or accessible from the aircraft in flight. Moreover they fulfill customary GPWS functions, an additional function of predictive alert of risks of collision with the relief or with obstacles on the ground consisting in alerting the crew of the aircraft when the short-term forecastable trajectory of the aircraft may encounter the ground or an obstacle on the ground and a function of display, on the instrument panel, of a map of the region overflown mentioning the threatening reliefs and obstacles on the ground.
In these TAWS ground proximity warning systems, a risk of collision with the ground is likened to the penetration of the relief in a set of disengagement trajectories, climbing at maximum slope, plotted from the current position of the aircraft, over a certain angular opening in the azimuthal plane about the course of the aircraft since these disengagement trajectories are a compulsory switch, as a last resort, for an aircraft seeking to attain a safety altitude.
Certain TAWS ground proximity warning systems, such as those described in American patents U.S. Pat. No. 5,442,556 or U.S. Pat. No. 5,892,462, actually calculate the elements most representative of the disengagement trajectories within the range of the aircraft from its current position and may therefore propose an avoidance trajectory when they detect a risk of collision with the ground. However, this avoidance trajectory is not necessarily the simplest to implement.
Most ground proximity warning systems of TAWS type have no explicit avoidance trajectory to propose to the crew since, to limit calculational requirements, they merely determine coarsely, one or more protection volumes tied to the aircraft, which extend in front of and beneath the aircraft and are dimensioned in such a way as to contain the majority of the disengagement trajectories within the range of the aircraft, vis-à-vis a possible relief or obstacle on the ground placed in its short-term forecastable trajectory.
There is therefore a requirement for procedures for computing an avoidance trajectory bringing an aircraft threatened by a risk of collision with the ground, into a secure zone where it can deploy freely flat, for the time required for the situation to be taken in hand again. The same requirement makes itself felt for an aircraft threatened with a risk of penetration into a prohibited zone. As an avoidance trajectory is defined mainly by the aim point which must allow the aircraft to access, under the best possible conditions, a zone of free lateral deployment, the problem of the computation of an avoidance trajectory boils down essentially to that of the selecting of a point of a zone of free lateral deployment, which is easy to access for an aircraft from its current position.